It is well known to employ changes in the dielectric constant between two capacitor plates as means for weighing various objects. U.S. Pat. No. 4,520,885 to Jeffrey, for example, describes a load sensing mat comprising a pair of superimposed spaced, conductive elements held together by resilient material so that the elements constitute capacitance plates which produce electric signal changes according to the load placed on it to compress the resilient material and thus vary the spacing between the capacitance plates.
U.S. Pat. No. 4,366,876 to Chen describes an hydraulic bellows type scale which has a plurality of hydraulic bellows, a variable capacitor and an integrated circuit and display unit. The weight of an object placed on the scale is transmitted by means of pressure exerted on the hydraulic bellows to the capacitor which produces a capacitance signal which reflects the weight of the object. U.S. Pat. No. 4,581,677 to Hruby et al. describes a layered capacitive sensor for detecting loads and load variations. The device comprises two outer protective layers, a first electrical conductor layer, a first electrically insulating layer, a second electrically conductive layer, a second electrically insulating layer and a third electrically conductive layer. The insulating layers are made of a resilient material to permit compression of the multilayered mat thereby producing a change in electric capacitance.
These devices of the prior art have, however, required that the distance between capacitor plates vary in relation to weight placed upon the plates thereby inducing a change in the capacitance, the change which is measured as an electric signal. Such systems have been found, however, to posses certain characteristics which result in their being inherently inaccurate and unreliable. Particularly where compressible materials are employed between capacitor plates, a large hysterisis effect is encountered in which the relative geometry of the capacitor does not remain constant for a given load. This effect results in inconsistent and unreliable readings since the critical distance between the plates cannot be maintained for a constant load.
Further, where a weight sensitive mat is being employed to measure snow cover, it is important that the thickness of the mat remain constant under variations in snow load temperature and barometric pressure changes. If the mat compresses and enlarges, the snow load itself is compressed and more prone to bridging and or fill in by more snow thereby giving an erroneous snow cover measurement.
Accordingly, it would be advantageous to provide a new and improved scale and methodology for weighing objects using capacitor plates in which the distance between the capacitor plates and the thickness of the mat containing the plates can be maintained constant irrespective the objects placed upon it.
It is a further object of the present invention to provide a weighing system for determining the weight of objects which employs a pneumatic bellows controlled by change in dielectric constant between capacitor plates to maintain a constant distance between capacitor plates irrespective of the weight placed upon the system.
Yet a further object of the present invention is to provide a pressure sensitive system which is inexpensive, versatile, and can be employed in a number of applications including the measurement of snow water content, rain water collection, and the depth of large bodies of water into which the device can be submerged.